Dye laser

ABSTRACT

A dye laser system wherein the dye medium is pumped by frequency doubled mode locked laser pulses from a Nd:YAG laser and also mode locked by colliding pulses in an anti-resonant ring having a saturable absorber.

The present invention relates to dye lasers, and particularly to a dyelaser system which allows the dye laser to be synchronously pumped by aCW mode-locked laser.

The invention is especially suitable for producing stable ultrashortlaser pulses, for example of sub-picosecond duration (90 femto-seconds(fs) with jitter in time of occurrence of less than 20 picoseconds).

It is a feture of the invention to provide a colliding pulse mode lockeddye laser which is synchronously pumped by a CW mode-locked laser, orfrom a CW mode-locked laser system which may include a laser amplifierpumped by a CW mode-locked laser oscillator, as described in a patentapplication, U.S. Ser. No. 593,992, filed Mar. 27, 1984 in the name ofSteven Williamson, et.al., and assigned to the same assignee as thepresent application.

Synchronous pumping of a colliding pulse mode-locked (CPM) dye laserfrom a mode-locked laser has been precluded because of phase (time)differences between the pump pulses and the pulses in the cavity of theCPM laser. It has been discovered in accordance with the invention thatsuch phase differences can be overcome by the use of an anti-resonantring in the dye laser cavity. Thus, there is provided a dye laser systemwhich is both synchronously pumped and colliding pulse mode locked,yielding stable ultrashort laser pulses.

The foregoing and other features, objects and advantages of theinvention will become more apparent from a reading of the followingdescription in a connection with the accompanying drawings in which:

FIG. 1 is a schematic diagram of a dye laser system provided inaccordance with the invention; and

FIG. 2 is a graph showing an output laser pulse provided by the systemshown in FIG. 1.

Referring to the drawings, there is shown a dye laser having a linearcavity 10. The cavity has four mirrors 12, 14, 16, and 18. The endmirror 12 is an output coupler (suitably 1%). The other mirrors 14, 16and 18 define the ends of an anti-resonant ring containing a saturableabsorber. Suitably the absorber is a jet 20 (10 um in thickness) ofDODCI. Such absorbers are discussed in Ultrashort Light Pulses, S. L.Shapiro, Editor, Springer-Verlag, Berlin, Heidelberg, N.Y. 1977, pps.40-65. Reference may be had to A. E. Seigman, IEEE J. Quant. Electron.QE-9, 247 (1973) for further information concerning the design of ananti-resonant ring.

The dye, suitably a jet 22 of Rhodamine 6G (Rh6G) is pumped by a CWpumped mode locked Nd:YAG laser oscillator 24, the output of which isfrequency doubled in a doubler crystal 26. For further information as toCW pumped Nd:YAG mode-locked laser oscillators, reference may be had toT. Sizer, et.al., IEEE J. Quant. Electron., QE-19, 506 (1983); and T.Sizer, et.al., Opt. Commun. 39, 259 (1981).

The dye is synchronously pumped by the pulses from the laser 24 whichare frequency doubled (suitably 90 ps pulses of 532 nm light at 100 MHrepetition rate). The average power of the pump pulses may be 1.5 W. Thepump pulses are directed to the Rh6G dye jet 22 by mirrors 28, 30 and32. The laser pulses generated the Rh6G in the cavity 10 are split by a50% splitter 34 and focused in the DODCI jet 20. The pulses recombine atthe splitter and interfere so that the anti-resonant ring acts as a 100%reflector. The collisions of the laser pulses occur in the antiresonantring, in that the pulses split, travel in opposite directions around thering, and collide in the absorber 20 where a standing wave patternexists. In the Rh6G medium, however, only one laser pulse exists at anyone time. Pulses progagate linearly through the dye gain medium and donot collide. It will be apparent that at any instant there can be onlyone dye laser pulse in the part of the cavity between the end mirror 12and the beam splitter 34, while there can be two pulses in theanti-resonant ring part of the cavity. Accordingly, the pumping pulsescan reach the dye 22 at the same time as the dye laser pulses, andsynchronous pumping occurs.

It will also be seen that the cavity length can be adjusted so thatsynchronous pumping occurs. This adjustment does not affect the CPMoperation in the ring because of the isolation at the combining point(i.e., the beam splitter 34). Therefore, there is provided bothsynchronous pumping and CPM operation in the dye laser system.

FIG. 2 shows that the system generates output pulses with anautocorrelation width of 130 fs., which for Gaussian shape pulsescorresponds to a pulse width of 90 fs. With Rh6G the wavelength of theoutput pulses is 615 nm. In actual tests, the output power was 60 mW,and the spectral width approximately 60 Angstroms. The output pulsestability has the stability of the pump laser 24 (about 1% rms). Tuningmay be provided for with a 2 um uncoated pellicle (not shown) in thecavity 10.

We claim:
 1. A CPM dye laser system which generates laser pulses andhaving a laser cavity, comprising: an anti-resonant ring in said cavitydefining a 100% reflector at one end of said laser cavity, and meansincluding a CW pumped mode-locked laser for pumping a laser dye gainmedium with pump pulses synchronous with the laser pulses generated insaid cavity, the length of said cavity defined between said ring andanother reflector at the end of said cavity opposite to said one endsaid length being adjustable so that synchronous pumping occurs.
 2. Thelaser system according to claim 1 wherein a beam splitter is disposedbetween end mirrors of said cavity, and said beam splitter defines partof said ring.
 3. The laser system according to claim 1 wherein saidanti-resonant ring has an absorber medium therein, and wherein saidpulses generated in said dye laser gain medium collide in said absorbermedium to provide CPM operation in said cavity.
 4. The laser accordingto claim 3 wherein said pumping means includes means for changing thefrequency of the pump pulses from said CW mode-locked laser.
 5. Thelaser system according to claim 3 wherein said ring includes a beamsplitter at which said dye laser pulses split to travel in oppositedirections around said ring to collide in said absorber medium.
 6. Thelaser system according to claim 5 wherein said absorber medium in saidring includes an absorber dye material.
 7. The laser according to claim6 wherein said CW mode-locked laser has an Nd:YAG gain medium, said dyeis Rh6G, and said absorber dye is DODCI.
 8. The system according toclaim 1 wherein said ring includes a saturable absorber medium.
 9. Thesystem according to claim 1 wherein said ring is defined by a beamsplitter through which laser pulses enter and leave and in which saidlaser pulses combine after CPM operation in said ring.
 10. The systemaccording to claim 9 wherein said cavity is defined by at least 4mirrors, at least 2 in said ring, and one being an end mirror.
 11. Thesystem according to claim 9 wherein said dye laser includes a dyemedium, said pumping means including means for directing said pumppulses from said mode-locked laser onto said dye medium.
 12. The systemaccording to claim 11 wherein said ring includes a saturable absorbermedium.
 13. The system according to claim 12 wherein said CW mode-lockedlaser has an Nd:YAG gain medium, said pumping means includes means fordoubling the frequency of the pump pulses from said Nd:YAG CWmode-locked laser, said dye medium is Rh6G, and said absorber is DODCI.14. The system according to claim 12 wherein said absorber and dye arein the form of jets.